Figure 224

Base pairings between a purine and pyrimidine base link the two polynucleotide strands of the DNA double helix.

TABLE 2-7 Comparison of DNA and RNA Composition

DNA

RNA

Nucleotide sugar

Deoxyribose

Ribose

Nucleotide bases

Purines

Adenine

Adenine

Guanine

Guanine

Pyrimidines

Cytosine

Cytosine

Thymine

Uracil

Number of chains

Two

One

DNA is replaced in RNA by the pyrimidine base uracil (U) (Figure 2-23), which can base-pair with the purine adenine (A—U pairing). The other three bases, adenine, guanine, and cytosine, are the same in both DNA and RNA. Although RNA contains only a single chain of nucleotides, portions of this chain can bend back upon itself and undergo base pairing with nucleotides in the same chain or in other molecules of DNA or RNA.

SUMMARY

Guanine Cytosine

FIGURE 2-25

Hydrogen bonds between the nucleotide bases in DNA determine the specificity of base pairings: adenine with thymine and guanine with cytosine.

FIGURE 2-25

Hydrogen bonds between the nucleotide bases in DNA determine the specificity of base pairings: adenine with thymine and guanine with cytosine.

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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